Eastern Sierra Snowpack Summary - 2016-03-07 07:47

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THIS ADVISORY EXPIRED ON March 10, 2016 @ 7:47 am
Avalanche Advisory published on March 7, 2016 @ 7:47 am
Issued by Doug Lewis - Eastern Sierra Avalanche Center
Avalanche Character 1: Wind Slab
Wind Slab avalanches release naturally during wind events and can be triggered for up to a week after a wind event. They form in lee and cross-loaded terrain features. Avoid them by sticking to wind sheltered or wind scoured areas.

Windslab exist throughout the region as the result of high intensity snowfall that began Saturday night into Sunday morning with strong Southwesterly winds. Another storm system is forecast to move into the area today (Monday) with strong SW winds (shifting to NW by afternoon) with gusts upwards of 100 mph over ridgetops with 1-2’+ of new snow forecasted.  Dangerous windslabs will form just below ridges and across exposed slopes and open forested terrain facing NW-N-NE-E-SE, especially at mid to upper elevations.  Natural wind slab avalanches are likely and human triggered avalanches very likely that could result in burial and death.

Avalanche Character 2: Storm Slab
Storm Slab avalanches release naturally during snow storms and can be triggered for a few days after a storm. They often release at or below the trigger point. They exist throughout the terrain. Avoid them by waiting for the storm snow to stabilize.

Storm slab avalanche concerns exist throughout the region as the result of high intensity snowfall that began Saturday night into Sunday morning and is forecasted to continue into Monday with an additional 1’-2’ of new snow adding additional load and stress to an already tender snowpack. The next round of precipitation will see cooler temperatures, with possible density changes within the new storm snow, which will likely increase avalanche concern for storm slabs in wind protected areas for slopes >32 deg facing all aspects.  Steep convexities are areas of obvious concern, as are gully bottoms that act as terrain traps with the potential for deep burial. Natural storm slab avalanches will be possible, and human triggered avalanches likely early during the forecast period, potentially easing slight by mid-week. 

Avalanche Character 3: Persistent Slab
Persistent Slab avalanches can be triggered days to weeks after the last storm. They often propagate across and beyond terrain features that would otherwise confine Wind and Storm Slab avalanches. In some cases they can be triggered remotely, from low-angle terrain or adjacent slopes. Give yourself a wide safety buffer to address the uncertainty.

In exposed, high elevation terrain (>11,500') that faces N to NE some areas of very faceted shallow snowpacks have been found to exist. The recent significant new snowload could very well overburden these weak areas and result in larger avalanches to the ground.  Natural and human triggered avalanches are likely in these isolated areas.  

Snowpack Discussion

Primary avalanche concern for this forecast period are: Storm Slab and Windslabs that formed during the intense snowfall that begun late Saturday and Sunday with additional heavy snows forecasted stating today (Monday).  

A strong storm system swept through the area Friday through Sunday morning, which brought strong Southwest winds aloft, some rain at the lower elevations (~ 7,500’ and below) and significant amounts of wet snow (trending toward drier early Sunday) fell over the weekend. Some impressive snowfall amounts of 2’ (June Mountain) and 3’ (Mammoth Mountain) in 24 hours as of Sunday evening with areas north of June, reporting nearly an 1” of water at around 7,000’ with snow depth stations reporting 1’ to 2’ of new snow with similiar amounts south of Mammoth towards Bishop. The system cleared the region quickly Sunday morning and in it’s wake initially light to moderate Southwest to Westerly winds. Winds picked up abruptly Sunday afternoon with extensive snow transport being reported throughout the region. Massive snow banners were evident off most peaks, with obvious wind eddies and crossloading of terrain features from multiple directions. Winds subsided a bit in advance of second short wave that made landfall late Sunday into Monday. This second system is cooler and again accompanied by strong Southwest flow.  Extensive avalanching was reported prior to the arrival of Monday's storm. Mammoth reported 1’ to 3’ crowns with extensive propagation, with some failing in the new snow as well as at the old/new snow interface. However, June reported minimal activity while outside the ski area avalanches were observed in the Negatives, and Dream Peak. Observers also reported evidence of naturals along Nevahbe ridge, McGee.

A Winter Storm Warnings is in effect for Mono County through 4PM Monday. 

Avalanche concerns are currently focused on widespread windslabs throughout the mid to upper elevations on open exposed slopes and open trees as well as storm snow failures. Sunday’s wind event clearly transported snow well downslope from the ridgetops and summits forming tender windslabs of varying thickness primarily on N-E-SE aspects but NW and South aspects maybe suspect too due to the shifting wind directions and varying wind velocities. Sunday’s windslabs have not had sufficient time to strengthen and are now covered by additional new snow or freshly formed windslabs. The snowpack is still adjusting to this rapid loading event as evidenced by Compression Test scores hovering around 11 (failing at the old/new interface).

With the recent heavy successive snowfalls there is an additional concern for storm slab failures. The old/new snow interface is showing signs of strengthening but the additional load added Monday will strain those bonds and the cooler snow may not bond as well to the underlying recent snowfall. This scenario has the potential to produce avalanches within the new snow and steeping down or failing at the old/new snow interface, producing a deep release. Either scenario should be avoided due to the potential for deep burial and/or traumatic injuries.   

During the first part of the week, natural avalanches are likely; human- triggered avalanches very likely, especially on windloaded slopes 35 degrees and greater. Travel in avalanche terrain not recommended. If electing to travel in avalanche terrain (greater than 32 degrees) you will need to exercise your best route finding and snow stability assessment skills, up as well as down, to avoid potentially dangerous terrain or situations. Towards mid-week, avalanche conditions may improve somewhat overall as storm snow settles and strengthens. However, moderate N-NW winds and ample snow available for transport will continue to form new windslabs primarily on NE-SE-SE-W aspects with Natural avalanches becoming more unlikely; human- triggered avalanches possible.

recent observations

Dream Peak Trees (3/6/16) Toured to the Dream Peak Trees to assess the new snow stability. Avalanche debris noted on Yost Lake and obvious avalanche debris piled-up on the uphill side of trees. Bedsurface old/new snow interface, crown visible under rock band that defines western edge of basin. Slide propagated widely along western edge sending debris through well into the trees. Compression Tests failing on the old/new snow interface, scores hovering around 11, Q2.

Mammoth Mountain (3/6/16): Widespread lower and mid-mountain avalanches, some natural during the night, and many as a result of hand charges and ski cuts this morning.  12-36" crowns, running full width of paths.  Some avalanches failing within density changes of new snow, and many failing at new/old snow interface. 

*Disclaimer: These are mostly in-bounds areas, and may not be representative of backcountry conditions.

Sunday PM strong SW/W winds extensive plumes (windslab forming and extending well down slope)

Snowfall Totals (3/6/16):

June Lake Ski Area 27”, 24” in 24 hours!

Mammoth Ski Area 44”, 36” in 24 hours!

Virginia Lakes ~14”

Gem Pass ~23”

Agnew Pass  ~15”

weather

* Winter Storm Warnings in effect for Mono County and the Lake Tahoe Basin through Monday 4PM.

 * Timing: heavy snow will continue through this afternoon with additional snow accumulations: 1 to 2 feet above 7000 feet with 6 to 12 inches between 6000 and 7000 feet. East of highway 395 in mono county...3 to 6 inches.

 * Winds: southwest 15 to 30 mph with gusts up to 45 mph. Sierra ridge gusts up to 100 mph.

Mon: Heavy snowfall is forecast  for the Sierra early this morning (Monday) as second winter storm pushes into the West Coast with precipitation forecasted to spillover into the eastern Sierra Monday morning. Forecast model soundings are still indicating a very unstable atmosphere with steep lapse rates that would be conducive to showers developing after 10AM. As the upper level trough drops south and east later Monday afternoon, the flow turns north/northeast into the southern Sierra resulting in moist upslope flow into Mono/Alpine counties. Wrap around bands that could result in a late afternoon burst of snow shower with snowfall likely persisting into the evening. There is potential for additional accumulations around 1 to 2 feet of snow above 7000 feet through late this evening in Mono County. Moderate Southerly winds (40’s) will shift to the Northwest and ease somewhat (20’s). Temps will remain cool, 20’s to low 30’s.

Tues-Wed: A break in the weather is expected on Tuesday, with weak ridge of high-pressure building overhead with mid and high clouds pushing in from the west with Northwest winds (teens to low 20’s), Temperatures rising into the mid 20’s to 30’s, possibly low 40’s Wednesday.  

Thursday: Models show a system moving into the region with the GFS a little faster and farther south than the ECMWF with another round of precipitation by Friday. 

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
  Monday Monday Night Tuesday
Weather: CLOUDY. SNOW IN THE MORNING...THEN SNOW SHOWERS LIKELY IN THE AFTERNOON. MOSTLY CLOUDY. SNOW SHOWERS LIKELY IN THE EVENING...THEN CHANCE OF SNOW SHOWERS AFTER MIDNIGHT. PARTLY CLOUDY. SLIGHT CHANCE OF SNOW SHOWERS IN THE MORNING.
Temperatures: 27 TO 33 deg. F. 14 TO 20 deg. F. 31 TO 38 deg. F.
Wind direction: SOUTH BECOMING NORTHWEST IN THE AFTERNOON. NORTH NORTHWEST
Wind speed: 40 TO 45 MPH WITH GUSTS TO 60 MPH , 20 TO 25 MPH WITH GUSTS TO 35 MPH IN THE AFTERNOON 20 TO 30 MPH WITH GUSTS UP TO 45 MPH. 15 TO 20 MPH WITH GUSTS TO 35 MPH.
Expected snowfall: 4 TO 8 in. 0 TO 1 in. 0 in.
Over 10,000 ft.
  Monday Monday Night Tuesday
Weather: CLOUDY. SNOW IN THE MORNING...THEN SNOW SHOWERS IN THE AFTERNOON. MOSTLY CLOUDY. SNOW SHOWERS IN THE EVENING...THEN CHANCE OF SNOW SHOWERS AFTER MIDNIGHT. PARTLY CLOUDY. SLIGHT CHANCE OF SNOW SHOWERS IN THE MORNING.
Temperatures: 20 TO 26 deg. F. 5 TO 12 deg. F. 26 TO 33 deg. F.
Wind direction: SOUTHWEST BECOMING NORTHWEST NORTH NORTH
Wind speed: 45 TO 50 MPH WITH GUSTS TO 75 MPH, DECREASING TO 20 TO 25 MPH WITH GUSTS TO 40 MPH IN THE AFTERNOON. 20 TO 30 MPH WITH GUSTS UP TO 50 MPH. 15 TO 25 MPH WITH GUSTS TO 40 MPH.
Expected snowfall: 6 TO 9 in. 0 TO 1 in. 0 in.
Disclaimer

This snowpack summary applies only to backcountry areas outside established ski area boundaries. This snowpack summary only describes general avalanche conditions and local variations always occur. This snowpack summary expires 24 hours after the posted time unless otherwise noted. The information in this snowpack summary is provided by the Eastern Sierra Avalanche Center who is solely responsible for its content.

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